This invention relates generally to separating components of a mixture that includes liquid and particulate components, and more particularly to apparatus and process for separating and recovering water components, liquified fatty components and particulate protein components of treatment mixtures produced from waste water treatment systems.
Most cities and counties have enacted requirements for industries which discharge water into the municipal sewer systems. These municipalities generally require the waste water to meet certain standards before being discharged into the sewer systems, and these standards usually relate to the level of pollutants which interfere with the efficient operation of the municipal waste water treatment facility.
Waste water which contains materials having a high biochemical oxygen demand, total suspended solids and ammonia increase the treatment cost to the city or county. The term "biochemical oxygen demand" ("BOD") refers to the quantity of oxygen utilized in the biochemical oxidation of organic matter; and the term "total suspended solids" ("TSS") refers to the total suspended solids which float on the surface of, or are suspended in, waste water and which are removable by filtering.
In some instances, a surcharge may be levied on an industrial plant that produces waste water which does not meet the standards as established by the municipality. For example, these standards may prohibit waste water containing greater than 300 mg/liter of BOD, 300 mg/liter of TSS, and/or 18 mg/liter of ammonia. Cities and counties may also prohibit industrial plants from discharging waste water having an oil, grease and fat ("FOG") concentration greater than 150 mg/liter.
A city or county also has the authority to revoke an industrial plant's waste water discharge permit if that plant continues to discharge waste water which does not meet the standards set by that city or county. Therefore, an industrial plant must effectively treat its waste water to reduce the level of pollutants and thereby meet such standards.
An effective waste water treatment process for an industrial plant should cause the solid organic and inorganic matter to flocculate and form a sludge cake. This sludge cake accumulates to a certain thickness and is then separated from the liquid component of the waste water and sent to a rendering plant for further processing. If the resulting sludge cake is under 20%-25% solids, the rendering plant may impose a surcharge because sludge with a high percentage of water is more expensive for the rendering plant to process.
One method of treating waste water from an industrial plant is disclosed in Stewart, U.S. Pat. No. 4,981,599. This process results in the formation of a sludge cake on top of the waste water. The sludge cake is removed, and the treated waste water is discharged into the sewer system.
However, the sludge cake (also referred to as a float material) produced by the Stewart process is not ideally suited for subsequent treatment at a rendering plant because (1) the water content of the float material is higher than desired and (2) the float material is comprised of fatty and protein components which desirably should be separated from the water component.
Conventional apparatus used for separating water components from a sludge cake or other mixtures produced from a waste treatment system typically include a pair of opposing, endless filtering screens constructed of a cloth material made of polypropylene, polyethylene or other similar material. For example, such apparatus are manufactured by Phoenix Process Equipment Company of Louisville, Ky., as Belt Filter Press, model WW-X, and by Ashbrook-Simon-Hartley of Houston, Tex., under the trademark KLAMPRESS. The filtering screens are driven in the same direction so that treatment mixture deposited between the screens is entrapped between the screens as the mixture is moved through the apparatus. Once the mixture is captured, the screens are conjointly routed through a series of compression rollers that pull the screens toward each other to squeeze the mixture between the screens. Water is thus squeezed from the mixture, leaving a cake-like mixture as an end product.
In co-assigned U.S. patent application Ser. No. 08/787,792, a novel process is disclosed for separating and recovering water, fatty and protein components of a waste treatment system float material and for providing the protein and fatty components with a high percentage of solids (i.e. a low percentage of water content). The process includes creating a treatment mixture comprising float material and a heated carrier material. The temperature of the mixture is in the range of about 77.degree. C.-99.degree. C. so that the fatty components are generally liquified. The heated mixture is then subjected to a separating process in which the components are separated and recovered from the initial treatment mixture.
While the conventional apparatus discussed above are effective at removing water from a cooled treatment mixture (e.g., in which the fatty components are solidified or at least semi-solidified), there are a number of disadvantages associated with the use of the apparatus. For example, the filtering screens of these apparatus are not constructed to withstand high temperatures, such as temperatures of about 77.degree. C.-99.degree. C. Rather, the mesh of these screens tends to deform when exposed to such temperatures, resulting in poor tracking of the screens through the compression rollers and poor filtering of the mixture.
In addition, the mixtures processed by the conventional apparatus are relatively cooled, such that the fatty components are more solidified. This makes it somewhat more difficult to squeeze water components from the mixture. To compensate, the compression rollers are arranged to apply substantial squeezing pressure to squeeze water from the mixture between the screens. This typically results in particulate components of the mixture being mashed into and entrained in the filtering screens, requiring a substantial amount of cleaning by a washer assembly. For example, conventional apparatus typically use about 40 gallons of cleaning water per minute to clean enmeshed mixture from the belts. The cleaning water becomes contaminated with the waste material cleaned from the belts and must be sent back through the waste treatment system for reprocessing. The excessive use of cleaning water results in substantial reprocessing costs because the charge is typically based on the volume of water which must be treated. Moreover, using two filtering belts and a substantial number of compression rollers also increases the complexity and cost of manufacturing and operating the apparatus.